High-speed optical communication systems are moving towards advanced modulation formats to improve transmission properties, e.g. optical reach, dispersion tolerance, spectral efficiency, and the like. In Phase Shift Keying (PSK) modulation, the phase of a carrier is modulated. Quadrature Phase Shift Keying (QPSK) utilizes four phase levels for modulation. Conventional direct-detection optical receivers cannot detect optical phase, but can detect phase differences between adjacent bits by converting these phase differences into power changes. As such, optical communication systems utilize Differential Phase Shift Keying (DPSK) and Differential Quadrature Phase Shift Keying (DQPSK).
For operational DQPSK transmission systems, both with a Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ) pulse shape, a differential quaternary precoder is required in the transmitter. The precoder circuit is configured to map input information onto the four phase states, i.e. the precoder converts input data to a format susceptible for the DQPSK format. Automatic realignment of two streams of data into a DQPSK optical modulator is required for high data rates, such as 100 Gb/s. Serial DQPSK precoders are known in the art, and this approach has been tested at 10 Gb/s.
However, serial precoders are too expensive, and difficult to impossible to build for high-speed rates up to 100 Gb/s and beyond. It is extremely difficult to synchronize both data using discrete gates and flip-flops at data rates higher than 40 Gb/s. It is close to impossible to control propagation delay in discrete logic and cables.
Also, for operational DQPSK transmission systems, data stream alignment is critical to ensure a phase lock between independent data streams that are combined during the DQPSK modulation process. Conventional realignment processes utilize serial data and the realignment is done by selecting cable length and phase shifters. However, such conventional processes are inapplicable to high-speed, parallel systems, such as DQPSK systems operating at high-speed rates (e.g. 50 Gb/s, etc.). DQPSK transmission systems are limited by the absences of automatic realignment processes.